High-Z semiconductor radiation sensors typically include a cathode electrode and at least one anode electrode. The cathode and anode are typically disposed on opposite ends of a detector medium. The detector medium is conventionally composed of cadmium zinc telluride (CdZnTe or CZT), mercuric iodide (HgI2), or the like. An electrical potential is applied to the cathode and the anode to create an electrical field between the cathode and anode.
Cadmium-Zinc-Telluride (CZT) sensors have emerged as leading candidates in room-temperature, large-volume, gamma-ray spectrometry for security, medical, industrial, and space applications. More recently another wide-bandgap material, mercuric iodide (HgI2), has been suggested as a potential candidate. The spectral resolution of these materials is strongly limited by several deficiencies (e.g., the poor mobility of holes, electron trapping, and extensive non-uniformities). Accordingly, various solutions have been proposed that link suitable electrode configurations with bi-parametric signal correction.
3-D Position-Sensitive Detectors (3DPSDs) are a form of high-Z semiconductor detectors that combine pixilation of an anode electrode with a measurement of amplitude and timing. This information is used to reconstruct a position at which the ionizing interaction occurred, and to correct the measurement on a voxel-by-voxel basis (voxel refers to volumetric pixel), thus compensating for the deficiencies. With this approach, energy resolutions better than 1% FWHM at 662 keV in CZT have been achieved, limited by the resolution of readout electronics. Additionally, employing the 3D correction method allows the use of lower-grade and larger size detectors in more applications.
In these sensors, ionizing radiation impinges upon the detector medium, which results in the generation of charge (electron-hole pairs) in an amount proportional to the energy of the ionization radiation. As a result of the electrical field generated between the cathode and the anode, the electrons drift towards the anode and the holes drift towards the cathode inducing electrical signals that are converted into voltages by charge amplifiers electrically coupled to the cathode and anode. In most cases, in order to improve the resolution of one of the two electrodes, the anode electrode is typically divided into an array of anodes, referred to herein as anode pixels. Each anode pixel in the array is generally connected to a dedicated charge amplifier.
Most high-Z semiconductor sensors, like those including cadmium zinc telluride (CdZnTe) or mercuric iodide (HgI2), have poor charge transport properties. Holes have very low mobility, and during the signal processing time they essentially do not contribute to the electrical signal. Electrons typically have better mobility and are essentially the only contributors to the electrical signal.
A signal induced in the cathode starts developing at the time of the interaction of ionizing radiation in the detector medium and ends once the electrons reach one or more of the anodes. At the output of the charge amplifier, the cathode signal appears like a ramp with slope VC/TC proportional to the ionized charge and duration TC, which is equal to the electron travel time (i.e. duration TC and amplitude VC of the ramp are proportional to the depth of the ionizing event in the detector medium).
One conventional approach for measuring the timing of the cathode signal consists of filtering the cathode signal with a fast unipolar shaper (in order to improve the signal-to-noise performance) and measuring the time tTH at the crossing of a threshold VTH. One drawback of this approach is the dependence of the timing measurement on the amplitude of the cathode signal. A correction of the measurement is required based on the measurement of the amplitude of anode and cathode signals. Due to the number of measurements involved, the resulting timing t0 is affected by error in an amount that depends on the ionized charge and number of anodes involved.
Another conventional approach for measuring the timing of the cathode signal consists of continuously sampling the cathode signal at the output of the charge amplifier and, once the signal exceeds a threshold, extrapolating the timing of the signal from the multiple measurements. The sampling can be done by using multiple analog memories and providing analog-to-digital conversions at the end of the measurement or by using multiple analog-to-digital conversions in real time. Some drawbacks of this approach are moderate filtering with consequently poor signal-to-noise ratio, continuous switching or digital activity required by the multiple sampling before and during the waveform, and the requirement of multiple analog memories and associated analog-to-digital conversions that can limit in the rate capability of the detector.
Typically, due to the size of the anode pixels (small pixel effect), an anode signal induced in collecting anodes essentially only appears when the electrons arrive in proximity of the collecting anodes, and the amplitude of the anode signal is, in a first order, proportional to the charge and independent of the electrons travel time (i.e. independent of the depth of interaction of the ionizing event). At the output of the charge amplifier, the anode signal appears as a step with amplitude VA proportional to the ionized charge.
In addition to the poor charge transport properties, high Z semiconductor radiation sensors are affected by charge trapping. Charge trapping refers to electrons that become trapped while moving towards the anodes. The resulting anode and cathode signals are affected by an amount which depends on the traveling time. The charge trapping effects cause considerable degradation of the spectral response of these detectors because the amount of charge measured depends on the depth of the ionizing event.
To compensate for the charge trapping effects the amplitude of each anode signal can be corrected by measuring the depth of the interaction of the associated ionizing event. The depth of interaction is typically measured using one of two techniques. In the first method, the cathode to anode amplitude ratio is measured and in the second method the cathode-to-anode time delay is measured.
The first method typically consists of measuring the amplitudes VC and VA of the cathode and anode signals, respectively, and calculating the depth of interaction from the ratio VC/VA. In some cases, to improve the signal-to-noise ratio one or both of the signals are filtered with unipolar shapers. This approach is generally effective as long as the charge reaching the anodes is collected by a single anode. However, if the charge is collected by more than one anode (due to charge sharing or multiple-interacting events), the charge from each anode must be measured, and the sum from all involved anodes must be used in the ratio. For some anodes, the charge can be very small and the measurement can be strongly affected by noise. As a result, the spectral performance for ionizing interactions that result in charge sharing or multiple interacting events can be considerably degraded.
The second method generally consists of measuring the delay TC from the timing t0 of the cathode signal (i.e. the timing of the event) to the timing tA of the anode signal (i.e. the time of arrival of the electrons to the anode). The anode timing tA is measured from the anode signal at the output of the charge amplifier. In some cases, to improve the signal-to-noise ratio, the anode signal is filtered with a fast unipolar shaper. The measurement of the cathode timing is typically more difficult than that of the anode timing because the ramp signal at the output of the charge amplifier for the cathode is very slow and initially very small in amplitude.